Interpretive Summary: This study reports on the effect of weather factors (temperature, humidity, solar radiation and wind speed) on the foraging activity of pollinating insects in an apple orchard, with special emphasis on two managed bee species, Osmia cornuta and Apis mellifera. Most pollinating insects, including A. mellifera, were mainly active at the central hours of the day, ,when weather conditions were most favorable for flight. O. cornuta was the only species with similar activity levels throughout the day. Flies flew to the flowers in the late afternoon and remained there for the night. However, they were not very active on the flowers, and thus their pollinating value was very low. The dependence of foraging activity on weather factors was estimated for O. cornuta and A. mellifera. O. cornuta was the only species able to fly under strong wind, light rain or relative humidity higher 90%. It was fully active at temperatures of 10-12 degrees C and solar radiations of 200 Watts/m2. A. mellifera was fully active at 12-14 degrees C and 300 Watts/m2. Because O. corntua activity is less dependent on weather, this species pollinates for longer periods, both daily and seasonally, and therefore it is a more reliable orchard pollinator.

Technical Abstract:
The foraging activity of pollinator insects in relation to weather factors (ambient temperature, solar radiation, relative humidity and wind speed) was studied in an apple orchard in Girona, NE Spain, with special reference to two managed bee species, Osmia cornuta and Apis mellifera. O. cornuta was the only pollinator whose activity at the flowers was approximately constant throughout the day. Most other insects, including A. mellifera and other bees, were mainly active during the central hours of the day. Flies, Calliphoridae, Muscidae, Tachinidae and Anthomyidae, were recorded mostly in the early morning and late afternoon because they used the flowers as night refuges, and their pollinating effectiveness was very low. A. mellifera activity was significantly dependent on temperature solar radiation and wind speed (stepwise multiple regression: adjusted R2 =0.478; p < 0.0005); O. cornuta activity was dependent on solar radiation (adjusted dR2 = 0.060; p = 0.02); and syrphid activity was dependent on temperature (adjusted R2 = 0.094; p = 0.02). These results were confirmed through video recording at one O. cornuta nesting shelter and one A. mellifera hive. In general, O. cornuta was active from 10-12 degrees C and 200 Watts/m2, and A. mellifera from 12-14 degrees C and 300 Watts/m2. Wind speed and relative humidity had a lesser effect on pollinator activity, but O. cornuta was the only bee species seen visiting apple flowers under strong wind (50 Km/h) or light rain (0.9 mm/hour; > 90% relative humidity). Because of its greater tolerance to inclement weather, O. cornuta pollinated apple flowers for longer periods (both daily and seasonally), than other flower visitors. Thus,its benefit as an orchard pollinator should be particularly noticeable in years with significant frequent inclement weather during flowering.